The wet milling of corn in the United States of America produces over 4 million tons of corn fiber each year. To date, this corn fiber has been used in the United States of America in animal feed applications. Corn fiber comprises, among other things, hemicellulose, which is also referred to as corn fiber gum and arabinoxylans. Corn hemicellulose potentially has commercial value far exceeding its use in animal feed applications. For example, it is believed that corn hemicellulose may be used as replacement for gum Arabic in applications such as beverage flavor emulsification. Additionally, it is believed that corn hemicellulose may be useful in applications such as film formation, thickening, emulsification, and stabilization of aqueous solutions and suspensions. Further, it is known that including corn hemicellulose and its limited hydrolysis products, the arbinoxylan oligosaccharides (AXOS) that have a degree of polymerization (DP) of 3-10, in foods and beverages may yield health benefits such as increased absorption of calcium and magnesium, reduced cholesterol absorption, lowered plasma cholesterol, decrease cholesterol accumulation in the liver, and desirable bifidogenic effects.
In view of the number of possible relatively high value applications for corn hemicellulose, it is no surprise that processes of extracting hemicellulose from corn fiber on a commercial scale have been and continue to be the subject of investigation and development activities. For example, U.S. Pat. No. 6,147,206 (Donner et al.) discloses a process for obtaining high molecular weight hemicellulose that involves a treatment with hydrogen peroxide during or after treating milled corn fiber with an alkaline solution, which is often referred to as the “alkaline hydrogen peroxide” or “AHP” process. The hemicellulose produced by the AHP process is highly soluble in water. The yield of hemicellulose from the AHP process is known to be within a range of about 35 wt % (24 hours of extraction at 25° C.) to about 42 wt % (2 hours of extraction at 60° C.). Chromatographic analyses in conjunction with molar mass detection conducted on AHP end product revealed that a bimodal molar mass distribution having a high molecular weight component (8.4-16.1×105 g/mole) and a low molecular weight component (1.1-2.1×105 g/mole). White, fluffy cellulose/arabinoxylan mixtures (CAX) were generated from the solid residues remaining after corn fiber gum production. CAX contains a significant amount of sugars, as revealed from L-arabinose, D-xylose, and D,L-galactose levels. Even CAX prepared under extreme AHP conditions (e.g., 1 hour, 100° C.), contain a significant amount of sugars (e.g., about 33 wt %).
Another known process of extracting hemicellulose involves using hydrochloric acid or sulfuric acid. Such processes, however, result in significant hydrolysis to monomers such that the extracted product is primarily used for fermentation to ethanol or organic acids. For example, it has been reported that treating corn fiber with 1.0% sulfuric acid at 121° C. for 2 hours resulted in a total sugar yield of about 63.3 wt %.
Yet another known process of extracting hemicellulose from corn fiber is known as the ammonia-explosion (AFEX) process, which involves treating cellulose-containing materials with liquid ammonia to increase the chemical and biological reactivity of cellulose. More specifically, the AFEX process comprises (a) contacting cellulose-containing material with ammonia for less than one hour at a treatment pressure in the range of about 140 psia to about 180 psia, and at ambient temperature, (b) explosively reducing the pressure from the treatment pressure to atmospheric pressure, and (c) separating the ammonia from the cellulose-containing material, thereby increasing the digestibility of the cellulose in the cellulose-containing material and increasing the availability of protein from the fibers and from within the cell walls of the cellulose-containing material. After the AFEX process, the resulting corn fiber was subjected to enzymatic digestion with a combined mixture of commercial amylase, xylanase, and cellulase cellulose components for use in ethanol production. The digested material comprised about 30-40 wt % glucose and oligosaccharides.
Still another known process of extracting hemicellulose from corn fiber involves sequential SO2-catalyzed steam explosion and enzymatic hydrolysis as part of an enzymatically mediated cellulose-to-ethanol process. The efficiency of converting the combination of cellulose and hemicellulose in corn fiber to monomeric sugars using a steam exploded fiber was as high as about 80%, whereas the conversion efficiency without the steam explosion before enzymatic hydrolysis was only about 15%.
Each of the foregoing known processes of extracting hemicellulose from corn fibers have limitations. High quality hemicellulose can be prepared by the treatment of combination of alkali and hydrogen peroxide. However, the cost of production is too high with alkali addition, acid neutralization, and ethanol precipitation. Sulfuric acid-based or hydrochloric acid-based extraction have been shown to cause extensive hydrolysis of hemicellulose to sugars, which tends to make the extracted material unsuitable as a high value product such as a gum replacer, a low-sugar natural soluble fiber, or low-sugar bulking agent. Tests of the ammonia explosion process resulted in less than 20% of the total fiber being solubilized along with there being a significant browning reaction and residual ammonia were found in the extracted hemicellulose. Additionally, the yield of hemicellulose from hot liquid or steam treatments to corn fiber is too low to be commercially viable.
In view of the foregoing, a need still exists for a process of extracting hemicellulose from corn fiber that has one or more of the following attributes: it can be readily implemented on a commercial scale, is relatively low-cost; can produce one or more relatively high value products such as gum replacers, low-sugar natural fibers, and/or low-sugar bulking agents.